To manufacture the precise structures or machines, parts (or components) are needed to be precisely measured.
Micrometers or Vernier calipers have generally been used to measure lengths, inner or outer diameters of the parts. These devices normally have measurement errors of approximately 10 μm for the lengths, inner and/or outer diameters of the objects, however they cannot guarantee precise measurement in case the length of an object gets longer and/or the diameters thereof gets smaller.
For example, in case the length of an object is more than 30 cm or its inner diameter is less than 1 mm, it is difficult to precisely measure with conventional vernier calipers. The most accurate method for measuring the object length is to use an interferometer that uses a stabilized wavelength laser.
Lasers having stable wavelengths have been used as standard measurement tools for the length. In addition, HeNe lasers having stable energy transitions in gas state or lasers having wavelengths locked to absorbing lines of tube-shaped gas cell containing stable helium-neon laser or gas in gas conditions can also be used for the length measurements.
These lasers can be used as standard tools for the length measurements without periodic calibration. Thus, wavelength-stabilized laser interferometers can accurately measure the length regardless of object length, e.g., with a precision degree of 1 μm, or better, 1 nm,
In addition, if the inner diameter of an object is small, the laser interferometer system can accurately measure even for the inner diameter of 1 mm or less by magnifying the shape of the object.
Recently, as the resolution of imaging devices has been increased up to degrees of high-density (HD), 4K and 8K, the laser interferometers will ensure more accurate measurement using images.
Since imaging devices such as cameras and microscopes consist of imaging optical systems that project images to imaging devices such as CCDs or CMOS, which convert images collected through the imaging surface into electrical signals, they may provide distorted images due to the aberration of the imaging optical systems therein.
For example, referring to FIG. 1, (a) is an original image and (b) demonstrates distorted portions observed by viewers, particularly, it shows more severe distortions at the outer portions thereof. To correct these distortions, high-cost imaging optical systems are used, but a certain level of distortions is inevitable.
Thus, it is not easy to accurately measure the inner and/or outer diameters of an object only based on images. Particularly, the image-based measurement makes it difficult to measure the outer diameter and the concentricity of the outer diameter.
Although Korean Patent Publication No. 10-2013-0135438 discloses an interferometer-based measurement system, this does not describe a measurement apparatus using an interferometer and an image, due to a light source emitting a phase-locked pulse laser light, an optical splitter dividing the emitted light from the light source into a first optical path and a second optical path, a first reference portion reflecting light which is divided by the optical splitter and travels along the first optical path, an optical transmission unit transmitting another light which is divided by the optical splitter and travels along the second optical path, to an object under test, an optical detection unit detecting light received by superimposing the reflected light from the first reference portion and the reflected light from the object.